I've just upgraded from a 7D to a 5D III. Will I get any noticable difference if I upgrade my 60mb/s compact flash cards to 90mb/s cards?

This varies wildly depending on your usage. If you're photographing sports, wildlife, or anything else that requires a large burst rate, then a 90mb/s card wouldn't hurt. I have six CF cards in my lineup, and only one of them is 90mb/s -- which is used for when I want to push the envelope in terms of burst performance. The rest are 60mb/s other than a single 30mb/s "emergency card" I keep just in case.

I shoot mostly landscape and architecture, so the 60mb/s are more than adequate for my needs. But I would recommend against using the SD slot unless you need to. It's not UHS compatible, and you'll definitely notice a difference there. I keep a 32gb SD card in my slot, but have my 5D only record to the CF and use the SD as a failover in case I run out of space and don't have time to switch cards.

You might get more images in a long high speed burst, and, if you have a fast USB3 reader, images will download faster.I only upgrade in the event that I go for a larger card. I just upgraded from three 60mb/sec 16mb cards to a 64gb 1000X card but its mainly because downloading of a large number of files will be faster.Since I have dual cards now, I have a built-in backup so large cards are not a concern.

You will see a difference if you shoot long bursts, but it isn't going to give you 50% more.

I agree with a previous poster that having one really fast card is a good way to go if you know that you will be using it.Note that there is no point in getting fast SD cards for the camera, the camera write speed is limited.

Take the SD card out completely. It was discovered that when an SD card is in the camera, the CF card will record only up to the max rate of the SD card. Oh so I've been led to believe.

That's only if you're recording to both slots at once. If you specify a slot to record to, then it will utilize that card to its full potential. I have a CF and SD in my 5D, but have the camera record RAW only to the CF card. The camera will only fall back to the SD when the CF fills up. I haven't had any problems with card speed using this setup.

It's useful as an, "oh sh-t, I don't have another card." You wouldn't think it happens a lot, but it happened to me when I was in the Caribbean, and grabbed my camera for some sunset shots. Didn't bother taking my bag, just cam and tripod, and completely forgot to check my available shots. Having the SD saved me a 10 minute walk back to our hotel room during a nice sunset. Yeah, UHS would be nice... but even with my 7D I'd likely only use an SD slot as a backup anyway.

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Thanks everyone. I had already heard about the limitation of writing to CF / SD at the same time, but having an SD card to use as an interim is a good plan. I shoot a lot of sport, so normally get down to about 100 shots to go and then look for a good point to switch cards. Having the SD might give me a bit of extra room to pick the card switch point.

Lexar 1000's look to be the way to go, although I'll probably go for 16GB's.